Mantle pseudo-isochrons revisited

The 2.0 Ga pseudo-isochron age inferred from the mid-ocean ridge basalt 207Pb/204Pb against 206Pb/204Pb diagram is re-examined on the basis of a statistical box model of mantle processes. Simple equations are presented which relate the pseudo-isochron age to the decay constants and distribution of heterogeneity ages in the model mantle. In turn this age distribution is simply related to the history of melting. The equations are in good agreement with results from mantle convection simulations. The equations are different from but related to, and more general than, those found previously for mean box models. While the pseudo-isochron age does not signify a mean age in the usual sense, in the model presented it is related to a “generalised mean” over the distribution of heterogeneity ages. If a constant melt rate over the Earthʹs history is assumed, a mean remelting time of 0.5 Ga is required.